1. Field of the Invention
The present invention relates to steam ejection systems and specifically to methods and apparatus for the thermally efficient regeneration of steam used in ejection systems.
2. Description of the Prior Art
Ejectors are widely used in industrial processes, such as vacuum flash cooling and vacuum drying, to remove aqueous liquid, in the form of vapor, from a process stream in an enclosed chamber. The steam ejector consists of a steam nozzle for discharging a high velocity jet of steam across a suction chamber connected to an enclosed flash evaporation space from which process vapor is to be removed. The evacuated vapor is entrained in the high velocity steam jet in a converging mixing section, and the mixture then passes through a diverging diffuser section where the mixed stream is thermo-compressed to the exit pressure of the ejector.
In prior art ejection systems, the ejecting steam is generated in a fuel-fired boiler, either expressly for the ejection system or for use in a turbine for generating power prior to its delivery to the ejection system. In either case, the mixed stream of exhausted ejecting steam and process vapors exiting from the ejector outlet is condensed in a condenser fed by ambient cooling water.
The pressure at which ejecting steam is supplied varies widely over the range roughly from 0.5 to 30 bars, depending on the ejection process and the source of the steam. Secondary steam from a power generating means tends to be at the lower end of the range.
Normally, part of the condensate, equal by weight to the mass flow rate of ejecting steam delivered to the ejector, is rerouted through a condensate pump back to the boiler as makeup feedwater. The remainder, which is equal to the mass flow rate of process vapor evacuated from the evaporation chamber, is usually rejected from the process.
For example, a typical ejection system may use, as a working fluid, dry saturated steam at an initial pressure of 2 bars and a corresponding specific enthalpy of 646 kilocalories per kilogram (kcal/kg). A typical condenser temperature is 33.degree. C., which corresponds to a specific enthalpy of 612 kcal/kg for the mixture leaving the steam ejector. Consequently, an enthalpy drop of 34 kcal (646-612=34) occurs during the work doing expansion of one kg of ejecting steam in the ejector.
Since the specific heat of the condensate at 33.degree. C. is 33 kcal/kg, the heat rejected to the condenser coolant is 579 kcal (612-33=579) for each kilogram of ejecting steam that is condensed. All of this heat must be restored to the condensate recycled to the boiler in order to generate one kilogram of saturated steam at 2 bars. Assuming a boiler thermal efficiency of 0.90, the actual heat energy input per kilogram of generated steam is (646-33)/0.9=681 kcal. Of this amount, approximately only 5% (34 kcal/kg) is employed in the work-doing enthalpy drop during expansion in the ejector, meaning that the remaining 95% represents total waste.
Because of the rapidly escalating cost of fuel, it has thus become imperative to improve the fuel-effectiveness of conventional ejection systems.